This invention relates, in general, to time interval counters and, more particularly, to a time interval counter that may be incorporated into an atomic clock and provide improved means to determine the time interval between an internally generated time signal and an external standard time signal.
An atomic clock is a device to generate the time of day, time signals, and, frequently, standard frequency outputs from highly stable, internal, atomic oscillators, such as atomic rubidium oscillators. In such atomic clocks, highly stable frequencies are generated by the electronic emissions experienced in the transition between two energy levels of an atom or molecule. Such transitions generally occur in the microwave region of the electromagnetic spectrum and provide a highly stable frequency reference to which the frequency of a voltage-controlled, crystal oscillator (VCXO) can be electronically locked. The high frequency stability associated with an atomic reference frequency is transferred to a voltage-controlled, crystal oscillator. Hydrogen, cesium, and rubidium frequency standards can be used in atomic clocks to stabilize, usually, a five MHz, or ten MHz, quartz crystal oscillator.
In portable atomic clocks, rubidium atomic oscillators are preferred because of their small size and their contribution to portability. By slaving a voltage-controlled, quartz, crystal oscillator to the frequency of atomic transition of rubidium, the tendency of the quartz, crystal oscillator to drift through aging and other environmental effects can be markedly suppressed. Temperature and pressure changes, however, affect the frequency of the rubidium standard, and the atomic clock runs faster or slower, depending upon the change. Portable atomic clocks are usable to transfer precise times from one location to another and to calibrate remotely located equipment which needs a precise time of day to operate reliably and as desired. Such uses include equipment needing "flying clocks", geophysical survey-positioning systems, bistatic radar systems, and precise time-keeping systems. It is thus desirable to determine the time interval between the time signals generated by such a clock to "set" the portable clock with a standard time and to measure and compensate for the clock's drift.
Time interval counters for atomic clocks are known; however, the known atomic clocks were capable of measuring time intervals to only .+-. 20 nanoseconds accuracy and of synchronizing an internally generated time signal with an external standard time signal to an accuracy of only .+-. 50 nanoseconds.